Flashing lamp circuits

ABSTRACT

A flashing lamp circuit includes a capacitor which is charged by a source of d.c. supply when the circuit is energised and switch means including a relay the operating coil of which is energised when the voltage across the capacitor attains a predetermined voltage. In addition there is provided a current sensing means including a resistor and a transistor for sensing the current flow through the lamp or lamps. The current sensing means is arranged to maintain the relay coil energised in the event of failure of the or one or more of the lamps.

United States Patent 1 Brown Jan. 1, 1974 FLASHING LAMP CIRCUITS [75] Inventor: Robert Hulme Brown, Pnmary ExammerRy Lake KingSmWUPOWThamCS, England Attorney-John C. Holman et al.

[73] Assignee: C.A.V. Limited, Birmingham,

England 57 ABSTRACT [22] Filed: July 14, 1971 A flashing lamp circuit includes a capacitor which is [2]] Appl 271,654 charged by a source of d.c. supply when the circuit is energised and switch means including a relay the oper- [30] F i Application Priority Data ating coil of which is energised when the voltage July 17 1971 Great Britain 33 692/71 across the capacitor attains a predetermined voltage. In addition there is provided a current sensing means [52] CL 315/200 A 315/225 315/232 including a resistor and a transistor for sensing the 51 1m. 0.. ndsb 37/03 Current flow hwugh the lamp Or laml The Current 58 Field of Search 315/200 A 225 232 Sensing means is arranged maintain the relay energised in the event of failure of the or one or more [56] References Cited of'the lamps' UNITED STATES PATENTS 5 Claims, 2 Drawing Figures 3,329,868 7/1967 Domann et a1, 3l5/2OO A PAIENTEUJAN 1 I974 FIG.|

RIS

FIG.2

FLASHING LAMP CIRCUITS This invention relates to flashing lamp circuits of the kind including a capacitor which is charged by a source of d.c. supply when the circuit is energised and switch means operable to effect illumination of a lamp when the voltage across the capacitor attains a predetermined value.

The object of the invention is to provide such a circuit in a simple and convenient form and in which in the event of failure of the or a lamp where a plurality of lamps are being controlled, an indication of the failure will be given.

According to the invention in a flashing lamp circuit of the kind specified said switch means includes a relay the operating coil of which is energised when the capacitor voltage attains said predetermined voltage, there being provided current sensing means for sensing the current flow through said lamp or lamps, said current sensing means being arranged to maintain 'said relaycoil energise in the event of failure of the or one or more of said lamps during operation of the circuit.

Two examples of flashing lamp circuits in accordance with the invention will now be described with reference to FIGS. 1 and 2 respectively of the accompanying circuit diagrams.

Referring to FIG. 1 there are provided terminals 10 and 11 for connection respectively to the positive and negative terminals of a source of do. supply. In addition, there is provided a p-n-p transistor T1 the emitter terminal of which is connected to terminal 10 by way of resistor R1 and the collector terminal of which is connected to terminal 11 by way of the operating coil 12 of a relay. The base terminal of transistor T1 is connected by way of a resistor R3 to a point intermediate a capacitor C and a resistor R4. The other end of the resistor R4 is connected to an intermediate line 13 and the other plate of the capacitor C is connected to terminal 10 by way of a diode DI, the cathode of the diode being connected to the terminal 10. Furthermore, the emitter terminal of transistor T1 is connected to the intermediate line by say of resistor R2.

The relay is provided with a pair of normally open contacts 14 one of which is connected to the intermediate line 13 and the other of which is connected by way of a resistor R5 to terminal 10. This contact is also connected to the emitter terminal of a p-n-p transistor T2 the collector terminal of which is connected by way of resistor R8, to the collector terminal of transistor T1. Moreover, the base terminal of transistor T2 is connected to terminal 11 by way of resistor R9 and to terminal 10 by way of resistor R6 and diode D2 connected in series, the anode of the diode being connected to the terminal 10. In addition, diode D3 is provided and this has its cathode connected to the anode of diode D1 and its anode connected to the collector terminal of transistor T2 The indicator lamps which are controlled by the circuit are shown at 15. In circuit with these lamps between the intermediate rail 13 and terminal 11 is a manually operable switch 16 and a warming lamp 17 is connected between the terminal 10 and the intermediate rail 13.

In use, when the switch 16 is closed the warning lamp I7 is immediately illuminated and the capacitor C commences to charge by way of diode D3. When the voltage across the capacitor attains a predetermined value transistor T1 conducts and the relay coil is energised to effect closure of the contacts 14. With the contacts 14 closed the lamps 15 are illuminated to their maximum extent ignoring of course, the voltage drop across the resistor R5. Moreover, the warning lamp is extinguished. With the contacts 14 closed the capacitor discharges by way of the diode D1 and the resistors R3 and R4. When the capacitor is discharged transistor T1 ceases to conduct and the cycle is repeated so long as the switch 16 is retained in the closed position.

During normal conditions transistor T2 which conducts as soon as the switch 16 is closed, ceases to conduct shortly after the contacts 14 have closed. This is because the. lamp current flowing in the resistor R5 makes the potential of the emitter of transistor T2 more negative than the base thereof. It should be pointed out that the current flowing in transistor T2 is insufficient when the switch 16 is closed, to cause a current of sufficient magnitude to flow through the relay coil to effect closure of the contacts 14. In the event that one of the lamps is faulty then the total lamp current will not attain the level necessary to switch off transistor T2 and the current flowing in transistor T2 and hence in the relay coil when transistor T1 becomes non-conductive will be sufficient to maintain the contacts 14 closed and thereby an indication of failure of the circuit will be given in that the warning lamp 17 will remain extinguished and the lamps 15 will remain illuminated.

Diodes D1 and D3 are provided for the purpose of ensuring that the circuit ceases to operate in its correct fashion in the event that transistor T2 fails and becomes open circuit. In this event capacitor C would not be charged when switch 16 is closed and in practice transistor T1 would switch on and off at a high rate giving an indication of the failure of the transistor T2 or some associated components.

FIG. 2 shows an alternative arrangement in which wherever possible like reference numerals are used as are used in the circuit of FIG. 1. With reference to FIG. 2 transistor T1 of the circuit of FIG. 1 is replaced by a pair of transistors T3 connected as a Darlington pair. The collectors of the transistors are connected to one end of the relay coil and a diode D4 is provided inter mediate the junction of resistors R1 and R2 and the emitter of the second transistor of the pair. Moreover the base of the first transistor of the pair is connected to a point intermediate a pair of resistors R10 and R11. The other end of resistor R10 is connected to terminal 10 whilst the other end of resistor R11 is connected to the cathode of zener diode ZDl the anode of which is connected to terminal 11.

A p-n-p transistor T4 replaces transistor T2 and has its collector connected to; one end of resistor R8 and also to the anode of a diode D5 the cathode of which is connected to intermediate line 13. The emitter of transistor T4 is also connected to the intermediate line 13 and the base of transistor T4 is connected by way of resistor R12 to a point intermediate the contact 14 and resistor R5 by way of a diode D6. In this circuit the resistor R5 is connected to line 13. The base of transistor T4 is also connected to terminal 11 by way a resistor R13. Moreover the base is connected to one end of a resistor R14 by way of a diode D7. The other end of resistor R14 is connected to terminal 11 by way of resistor R15 and to intermediate line 13 by way of diode D8. In addition the anode of Zener diode ZD2 is connected tothe junction of resistor R14 and R15, and the cathode of Zener diode ZD2 is connected to terminal 10. Furthermore, line 13 incorporates a resistor R16, this being arranged intermediate the cathode of diode D and resistor R4b. Line 13 is fed from the negative terminal by way of resistor R15 anddiode D8 and Zener diode ZD2 fixes the potential of the line 13 relative to the positive terminal 10.

When switch 16 is closed then the chain of R16, R2 and R1, is brought into operation with ZD2 holding the potential across R1 and R2 this chain raises the emitter potential of the first transistor of the Darlington pair T3. The capacitor charges up through R4 until the potential across R is sufficient to switch T3 on. The relay contacts then close illuminating the lamps and the capacitor then discharges through the parallel paths of RIO/R3 and R4 until T3 switches off and the cycle repeats.

When transistor T4 switches on during the ON period of the lamps it keeps the relay closed and hence the warning lamp extinguished, the base of transistor R4 is biased through the chain of R13, R12 and D6. If the voltage across R5 falls to a sufficiently low value than T4 switches on The voltage across R5 will be sufficiently low if a lamp has failed and the current through R5 drops. The voltage at the base of T4 is also partly derived from the voltage across ZD2 via R14. This means that the voltage at the base of T4 does not vary linearly with supply voltage. This is to compensate for the variation of lamp current with supply voltage.

ZDl and R11 serve to switch on T3 in the event of high voltage surges thus preventing T3 from being broken down. D4 protects T3 against reverse voltages. D6 compensates for variations in base-emitter voltage of T4 with temperature. D5 protects T4 against reverse voltage. D7 protects T4 against reverse base emitter voltage when the contacts are open.

Resistor R8 ensures that the voltage developed across the relay coil due to T4 is only sufficient to hold the relay closed. The diode across the relay coil absorbs the switch off surge of the relay coil.

I claim:

1. A flashing lamp circuit for causing intermittent operation of a plurality of parallel connected lamps, said circuit comprising a relay, a pair of normally open contacts on said relay, said contacts being connected in series with said lamps and a source of electric supply current sensing means for sensing current flow through said lamps, a relay coil on said relay, a resistor through which the current sensing means can supply a holding current to said relay coil in the event the current flow to said lamps when said contacts are closed, is below a predetermined value indicative of failure of one or more of said lamps, a timing circuit operable intermittently to supply actuating current to said relay coil to effect operation of the relay and closure of the contacts, said relay being held in the operated condition in the event that the current flow to the lamps is below said predetermined value when said actuating current ceases to flow.

2. A circuit according to claim 1 in which said timing circuit comprises a first transistor having its emitter collector path connected in series with the operating coil of the relay a capacitor and means for charging the capacitor, said first transistor being responsive to the voltage across said capacitor.

3. A circuit according to claim 2 in which said current sensing means comprises a second resistor through which current flows to said lamps when said contacts are closed, and a second transistor responsive to the voltage developed across said second resistor, said second transistor when conductive supplying said holding current to the operating coil of the relay.

4. A circuit according to claim 1 including an indication lamp connected in parallel with a circuit including said pair of relay contacts.

5. A circuit according to claim 2 in which the charging current for the capacitor flows through said lamps. 

1. A flashing lamp circuit for causing intermittent operation of a plurality of parallel connected lamps, said circuit comprising a relay, a pair of normally open contacts on said relay, said contacts being connected in series with said lamps and a source of electric supply, current sensing means for sensing current flow through said lamps, a relay coil on said relay, a resistor through which the current sensing means can supply a holding current to said relay coil in the event the current flow to said lamps when said contacts are closed, is below a predetermined value indicative of failure of one or more of said lamps, a timing circuit operable intermittently to supply actuating current to said relay coil to effect operation of the relay and closure of the contacts, said relay being held in the operated condition in the event that the current flow to the lamps is below said predetermined value when said actuating current ceases to flow.
 2. A circuit according to claim 1 in which said timing circuit comprises a first transistor having its emitter collector path connected in series with the operating coil of the relay, a capacitor and means for charging the capacitor, said first transistor being responsive to the voltage across said capacitor.
 3. A circuit according to claim 2 in which said current sensing means comprises a second resistor through which current flows to said lamps when said contacts are closed, and a second transistor responsive to the voltage developed across said second resistor, said second transistor when conductive supplying said holding current to the operating coil of the relay.
 4. A circuit according to claim 1 including an indication lamp connected in parallel with a circuit including said pair of relay contacts.
 5. A circuit according to claim 2 in wHich the charging current for the capacitor flows through said lamps. 